1. Field of the Invention
The present invention relates to control systems for industrial plants.
2. Background of the Invention
There is often a requirement in industrial plants for a control system which will shutdown the industrial plant, should the industrial plant operate outside a predetermined operating condition regime.
A known control system which will shutdown the industrial plant, should the industrial plant operate outside a predetermined operating regime, comprises a plurality of sensors which detect parameters of the industrial plant and which send output signals to a logic circuit which analyzes the output signals to determine if the industrial plant is operating outside the predetermined operating regime. If the logic circuit determines that the industrial plant is operating outside the predetermined operating regime, a shutdown signal is produced which is sent to some means for shutting down the industrial plant.
However, the consequences of a failure to shutdown the industrial plant when required, i.e. when the industrial plant is operating outside the predetermined operating regime, are very severe especially in some chemical plants or nuclear reactor power plants. This known control system is inadequate since a failure of any part of the control system may allow the industrial plant to continue operating when shutdown is required.
To overcome the problem of failure of any part of the control system described, on improved control system comprising a number of sets of sensors, a number of logic circuits, and a voting logic circuit have been devised. In this control system; each set of sensors sends its output signals to one of the logic circuits, which analyzes the output signals to determine if the industrial plant is operating outside the predetermined operating regime. Each logic circuit sends a signal to the voting logic circuit. The voting logic circuit performs a function on the signals received from the logic circuits to produce a high reliability shutdown signal. If any of the sensors, or logic circuits fail, the remaining sensors in other sets and logic circuits ensure that shutdown signals are sent to the voting logic circuit by some of the logic circuits.
However, the voting logic circuits are susceptible to failure, and in some industrial plants the failure probability of the voting logic circuit is still too high in terms of the probability of failure to shut down when required.
A further control system to overcome the problem of failure of the voting logic circuit, has used a number of voting logic circuits each of which receive the output signals from all the logic circuits. In this control system each voting logic circuit operates independent means for shutting down the industrial plant.
Such a control system provides an effective system with considerably reduced probability of failure to shut down the industrial plant when required. However, this control system uses a relatively large number of voting logic circuits and a relatively large number of connections, which makes the control system relatively bulky, relatively complex and relatively expensive.